Evaporation, the process by which a liquid or a solid changes into a vapor. A substance may evaporate in one of two ways: (1) by changing into a vapor at the surface, as when water evaporates from an uncovered dish; or (2) by boiling, that is, by changing to vapor both within the substance as well as at its surface. A solid can evaporate by melting into a liquid, which then evaporates; or by changing directly into a vapor, or subliming. The rate of evaporation of a substance depends on its surface temperature, the pressure, and the humidity.

How Evaporation Occurs

Evaporation is caused by heat, the energy of molecules in motion. Molecules are motionless at absolute zero (-459.67 F. [-273.15 C.]). Above that temperature they are in constant motion, and the higher the temperature the faster they move. In moving, the molecules bump into each other. With each collision they transfer energy. If a surface molecule is bumped by two or three others in quick succession, it may gain enough energy to escape into the air, or evaporate. An evaporating molecule takes energy with it. Loss of this energy cools the substance from which it escapes.

Uses of Evaporation

Evaporation of perspiration cools our bodies. Evaporation of water from the earth keeps the air moist and provides the moisture for clouds. Thus evaporation has an important influence on weather and climate. It is important also in air conditioning, refrigeration, and distilling. Substances are evaporated in a partial vacuum to speed evaporation or to prevent deterioration by heat. Instant coffee and frozen orange juice are evaporated in this way, and so are blood plasma, vaccines, and antibiotics such as penicillin. Camphor is purified by sublimation.